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Experiment: Basic Solubility Rules of Salts in Aqueous Solutions

By jba102010 Apr 08, 2012 488 Words
The primary focus of experiment 4 was to teach a student the basic solubility rules of salts in aqueous solutions. After developing and using a scheme for the qualitative analysis of three cations in an aqueous mixture, a student would use a centrifuge to identify, precipitate, and separate the three ions in the mixture. After becoming familiar with all three cations, the student would use the scheme again to identify at least two of the cations in an unknown solution. In order to begin the separation process, HCl is added to the mixture of cations to form the precipitates and separate the chlorides of Ag+, Pb2+, and HG22+. It is important not to add too much HCl because an excess would form the soluble complexes of AgCl and PbCl2. After the HCl is added, it is centrifuged to form a solid precipitate that contains all three cations and separate them from the supernatant liquid. Finally, another drop of HCl is added to test for completeness of precipitation and centrifuged again if the solution remains incomplete.

To separate the lead (II) ion from the sample of solution, it must be separated from the insoluble Hg2Cl2 and AgCL salts by dissolving it in hot water. Once it has been dissolved from the other two cations, it is centrifuged and the liquid is decanted into another test tube. Once in the other test tube, a drop of 6.0 M acetic acid and two drops of 1.0 M K2CrO4 is added to the saved liquid. If the lead (II) cation is present, a yellow precipitate forms and ensures the presence of that cation. When testing my unknown solution, the lead (II) ion was not present in my solution. To further separate the cations, 1.0 mL of 6 M NH3 is added to the saved precipitate. After adding the ammonia, the mixture is centrifuged and the liquid is once again decanted into another test tube. If a gray or black precipitate forms, this proves the presence of Hg22+. The mercury (I) ion was present in my unknown.

Finally, separating the silver ions from the sample solution mixture is accomplished in the prior step when the mercury ion is centrifuged. In order to test for the presence of silver ions, 1 mL of 6M HNO3 is added to the solution from the prior step. The silver precipitates in this acidic solution. If a white precipitate appears, silver is present in the solution. The presence of a white precipitate in the solution proves that there were silver ions in my unknown. By following the scheme for the qualitative analysis, it was simple to separate and test for the presence of the target cations. It was critical to correctly label every one of the liquids to ensure the correct reactions. No other difficulties prevented success in the experiment, but performing the experiment several times would further reinforce the outcome of the experiment.

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